Einstein Probe detection of an X-ray Transient EP250815a

ATel #17338; C.-Y. Dai (NJU), W.-J. Zhang, H. Sun (NAO, CAS) report on behalf of the Einstein Probe (EP) team
on 16 Aug 2025; 10:40 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of an X-ray transient by EP-WXT through ground data analysis, designated EP250815a. The transient was first detected in the observation starting at 2025-08-15T13:32:37 (UTC) and exhibited a continuing flux increase through the most recent observation at 2025-08-16T02:20:20 (UTC). The WXT spectrum obtained from the latest observation can be fitted with an absorbed power-law model, giving a photon index of 2.1 (-1.1, +1.3) and a column density of 2.2 (-0.9, +1.3) x 10^22 cm^-2. The derived unabsorbed flux in 0.5-4 keV is 1.3 (-0.7, +4.2) x 10^-9 erg s^-1 cm^-2. The WXT position of the source is R.A. = 274.217 deg, Dec. = -19.643 deg (J2000), with an uncertainty radius of 2.2 arcmin (90% C.L. statistical and systematic).

We have scheduled a follow-up observation with EP-FXT, and will provide further updates as the analysis progresses.

The EP contact TA for this transient is Cuiyuan Dai (cydai@smail.nju.edu.cn).

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.